1. Field of the Invention
This invention relates to a method for and/or optical switching in a waveguide, particularly as activated by nonlinear optical signals and the optical switch so employed.
2. The Prior Art
Nonlinear optical processes have been the basis for many types of information processing. A component of these nonlinear optical processes is known as second harmonic generation (SHG). SHG has been observed in crystalline and oriented materials for over 30 years. Numerous geometries have been predicted and tested. Two features must be present in a material for efficient SHG production. These are a nonlinear susceptibility which produces the SHG and phase matching which allows the SHG that is produced from one region of the material to be in phase with the SHG that is produced with other regions of the material. If there is no phase matching a small amount of SHG can be observed from a material but most of the SHG light is destroyed by interference of SHG light from one region with another. So only with phase matching will all the peaks and valleys of the SHG fields produced in different regions line up and give efficient output signals.
Nonlinear optical signals can be produced in crystalline or poled waveguide materials. This is allowed because these materials can be manufactured so that they have the two key features of nonlinear susceptibility and phase matching. These crystalline and poled materials can have a nonlinear susceptibility which is uniform along the waveguide path. In this situation SHG can be phase-matched only by a change in the order of the waveguide mode and possibly the polarization. When the fundamental beam propagates in another mode it may produce some SHG but it will not be phase-matched and therefore the output will be small.
Another type of material can have a nonlinear susceptibility but initially no phase-matching capability. This material can be made into an efficient NLO material by periodically modifying the material along the waveguiding dimension so that phase matching can occur. One way to achieve this modification is a periodic modulation of the nonlinear susceptibility along the direction of propagation. Periodic modulation can occur in crystals, poled materials and in optically modified materials. Optically modified materials can have an intrinsic NLO activity or the NLO activity can be optically induced.
Also in the prior art is U.S. Pat. No. 5,253,258 to Lawandy (1993) which discloses an optically encoded phased-matched SHG waveguide made of certain doped glasses and capable of self-frequency doubling of an input beam. However, there is no indication of employing two modes in the same waveguide for optical switching purposes.
The theory of frequency doubling of laser beams in certain channel waveguides is discussed at length in Frequency doubling in Ti:MqO:LiNbO.sub.3 channel waveguides by F. Laurell, J. Opt. Soc. Am. B/Vol. 5, No. 2 Feb. 1988, which Article is incorporated herein by reference.
Again while frequency doubling in certain waveguides is disclosed, there is no indication of employing two modes in the same waveguide for optical switching purposes.
Also one device used for the interference of optical signals is called a Mach Zender interferometer. In this device a laser beam that is propagating in a confined waveguide such as a fiber or channel waveguide on an integrated circuit, is split into two channels. One of the channels is modified by an external device that changes the propagation time down the channels. When the two beams are then recombined they destructively interfere if one of the beams has its phase shifted by 180 degrees related to the other beam. This is an electro-optic switch. However, laser beams in waveguides are very sensitive to temperature changes and a slight temperature difference between the channels can distort the phase shift between such beams and reduce the interference therebetween, once recombined, and cause inaccuracies in the above electro-optic switch. Also is indicated, the Mach Zender interferometer requires two channels and thus added space.
Thus there is a need and market for an optical switch that reduces or obviates the above prior art shortcomings.
There has now been discovered a waveguide all optical switch wherein a pair of pulsed, laser beams are coupled into a waveguide to output a strong SHG beam in contrast with inputting but one of the laser beams into such waveguide, which outputs a weak SHG beam, to provide an on-off optical switch per the invention. As the invention requires but one waveguide (having two or more modes therein,) it is more compact and takes less space than the Mach Zender interferometer which requires two waveguides. Also as two modes in the same waveguide can be maintained at/or near the same temperature, the optical switch of the invention has higher accuracy and reliability then previously available.